This invention relates generally to variable delivery pumps within hydraulically-actuated systems, and more particularly to a method and structure for compensating for temperature changes in such pumps.
In several diesel engines today, variable delivery fixed displacement pumps supply pressurized actuation fluid to hydraulically activated systems within the engine. In one example high pressure common rail supplies pressurized lubricating oil to a plurality of hydraulically-actuated fuel injectors mounted in a diesel engine. The common rail is pressurized by a swash plate type pump that is driven directly by the engine. The rotation of the swash plate causes a plurality of parallel pistons to reciprocate up and down. The desired rail pressure is controlled at least partially as a function of the engine""s operating condition. At high speeds and loads, the rail pressure is generally desired to be significantly higher than the desired rail pressure when the engine is operating at an idle condition.
There are varying methods that pumps control the pressure within the common rail. For instance, variable delivery fixed displacement pumps such as shown in U.S. Pat. No. 6,035,828 issued to Anderson et. al on Mar. 14, 2000, control the pressure within the common rail by controlling the positioning of individual sleeves that are mounted to move on the outer surface of the individual pistons within the pump. When the engine requires a maximum amount of rail pressure, the individual sleeves are positioned such that they block fluid communication between a low pressure area and spill ports defined by the individual pistons. Thus, the fluid within the pumping chambers of the pistons is pressurized and displaced to the common rail. On the other hand, when the engine does not require high rail pressure, the individual sleeves are positioned such that the spill ports are in fluid communication with the low pressure area. Thus, virtually all of the fluid is displaced to the low pressure area rather than being pressurized within the pumping chamber. The individual sleeves can be positioned at different points along the outer surface of the respective pistons in order to achieve a desired output between maximum and minimum outputs, and hence a desired rail pressure.
While variable delivery fixed displacement pumps controlling rail pressure through the positioning of sleeves have performed well, there is room for improvement. For instance, at lower temperatures, the viscosity of the oil increases, requiring increased force to move the sleeve. This reduces the actuator response of the individual sleeves and may result in long cranking cycles or the inability to start the vehicle or machinery. Engineers have address this problem by widening the clearance between the individual sleeves and the pistons in order to reduce the oil sheared between the piston and the individual sleeve while the parts more relative to one another. By widening the clearance, less force is required to move the sleeve and actuator response increases. On the other hand, at higher temperatures, the viscosity of the oil decreases, requiring less force to move the individual sleeves. If the clearance between the individual sleeve and the piston is too wide, there is increased leakage of oil through the clearance into the low pressure area, which also may result in decreasing pump efficiency. Thus, engineers have been forced to design the sleeves such that the clearance between the sleeves and the pistons is a compromise that allows acceptable performance between high and low temperatures, but no exceptional performance at any temperature.
The present invention is directed to one or more of the problems set forth above.
In one aspect of the present invention, a pump includes a pump housing defining an inlet and an outlet and in which a barrel is at least partially positioned. At least one piston reciprocates in the pump housing and is at least partially positioned in the barrel. A moveable sleeve surrounds an annular outer surface of each piston. At least one of the sleeve and the barrel is made of a material with a lower coefficient of thermal expansion than the material out of which the piston is made.
In another aspect of the present invention, there is a method of compensating for temperature change within a pump. When the temperature within the pump is low, the clearance between an annular outer surface of a piston and at least one of an inner surface of a sleeve and a barrel is increased. When the temperature within the pump is high, the clearance between the annular outer surface of the piston and at least one of the inner surface of the sleeve and the barrel is decreased.